1. Field of the Invention
This invention relates to polymer blend, hollow fiber membranes having improved mechanical properties by alteration of the thermodynamic phase equilibrium of the concentrated polymer blend spinning solution through the incorporation of organic or inorganic complexing agents.
2. Description of the Related Art
The science of polymer blend miscibility and the prediction of the types of phase diagrams for polymer blends in solutions are quite complex. Unusual two-peaked coexistence curves and phase diagrams with a tendency toward greater miscibility at intermediate temperatures are reported in the literature for different polymer blends. The following articles and textbooks depict the complexity of the theory of polymer blend miscibility and provide several examples of different thermodynamic phase diagrams for polymer blends:
R. L. Scott, J. Chem. Physics, Vol. 17, p. 279 (1949); PA1 D. R. Paul & S. Newman, Polymer Blends, Vols. 1 and 2, Academic Press, San Francisco, London (1978); PA1 O. Olabasi et al., Polymer--Polymer Miscibility, Academic Press, New York (1979); and PA1 Pierre-Gillesde Gennes, Scaling Concepts in Polymer Physics, Chapter 4, Sections 4.1 and 4.2, Cornell University Press (1979).
Polymer blends which are molecularly compatible in the solid state at temperatures below the glass transition temperature of the blend might exhibit a thermodynamic phase equilibrium consisting of a coexistence curve in the solution state. The coexistence curve is the locus of the critical solution temperature (CST) as a function of the blend composition.
A particular blend solution might exhibit a coexistence curve for the higher critical solution temperature (HCST) or for the lower critical solution temperature (LCST), or for both the HCST and the LCST. For any specific blend composition, the blend solution is two-phase if the temperature is below the HCST or if the temperature is above the LCST (assuming that the phase diagram contains coexistence curves both for the HCST and LCST). For that type of phase diagram, the blend solution is single phase for any specific blend composition if the blend solution temperature is above the HCST and below the LCST.
U.S. Pat. No. 5,047,487 issued to Camargo et al. discloses that ULTEM 1000, a polyetherimide available from GE, and MATRIMID 5218, a phenylindane-containing polyimide available from Ciba, are molecularly compatible in the solid state. The molecular scale compatibility of the two polymers over the entire blend composition range was characterized by Camargo et al. utilizing the technique of Differential Scanning Calorimetry (DSC). The ULTEM/MATRIMID blends exhibit a single glass transition temperature (T.sub.g) located in between the T.sub.g of the individual blend components over the entire blend composition range, which indicates molecular-scale blend miscibility.
U.S. Pat. No. 5,085,676 issued to Ekiner et al. discloses solution spinning of hollow fiber membranes from concentrated ULTEM/MATRIMID blend solutions. U.S. Pat. No. 5,443,728 discloses membranes prepared from blends of polyetherimide and phenylindane-containing polyimides.
In prior practices, blend solutions have been stored above the HCST to ensure phase homogeneity. This may unfortunately result in polymer degradation reactions due to sensitivity of the blend polymers to high temperature exposure for prolonged times. The degradation of the blend polymer adversely affects the final product properties.
On the other hand, storage at temperatures below the HCST results in a two-phase solution. With prior methods, additional processing steps for heating and mixing of the two-phase blend solution were needed for formation of a homogenous blend solution prior to the final processing step. A two-phase blend polymer phase morphology in the solution state would also adversely affect the fiber spinning process continuity and the final product properties.
ULTEM and MATRIMID polyimide solutions are particularly susceptible to molecular weight degradation reactions in the solution state when exposed to elevated temperatures for prolonged times.
It is, therefore, an object of the present invention to provide a polymer blend that does not suffer from the disadvantages mentioned above. In particular, it is an object of the present invention to depress the HCST of a polymer blend solution in order to enhance its phase stability during low temperature storage prior to processing. It is a further object of the present invention to depress the HCST of the ULTEM/MATRIMID blend solution formulations in order to permit storage of the solutions at lower temperatures without phase separation prior to solution spinning into hollow fiber form.
These and other objects of the invention will become apparent in light of the following specification, the figures, and the claims appended hereto.